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Abstract
This paper describes how multiphase flow numerical simulation was applied to an exploration well, drilled offshore Australia to:

1. Predict and provide a better understanding of well behaviour during initial clean-up and testing operations for the planned flow rates and build-up sequence far in advance.

- Verify/Modify flow rates, fluid sampling and build-up schedules.

2. Use the P-T and rate data recorded during well testing to validate the well model and obtain more accurate downhole multiphase rates (back-calculate production rates from high resolution surface and downhole P-T) to optimise the application of deconvolution techniques, leading to increased radius of investigation.

The challenging well location environment and flow assurance issues (slugging, hydrates, etc.) can significantly extend the planned duration of well-testing and affect the value of the recorded data. The well testing rate schedule encompassing the initial, second and main flows and build-ups were evaluated, investigating the multiphase flow rates, P-T and liquid hold-up when opening the well to the MODU and shutting-in downhole. The steady state flow conditions for each gas rate were verified or changes recommended. The segregation of fluids remaining in the wellbore after well shut-in and its accumulation on top of the downhole valve were also modelled, including its effect in the following kick-off flow phase. A 500 psia underbalanced pressure with the wellbore full of brine and diesel was the initial condition for the simulation. LCM sensitivities were included to assess the effect of back-producing mud lost into the formation during drilling. The post-test flow-build-up transient data recorded was used to validate the model and optimise downhole multiphase rate accuracy to optimise the application of deconvolution techniques.

The primary technical contribution of this case study is the application of Dynamic Simulation to refine well testing procedures and optimise data accuracy for well test analysis.

Dynamic Simulation is a useful tool that can be used to "virtually" run through a complete well-test operation to validate and optimise well testing design, planning and operations, as well as improving data and multiphase downhole flowrates accuracy for pressure transient analysis.

Introduction

Annular pressure operated DST tools with HiPack packer and 4-1/2” TCP guns were run on a 3-1/2’ OD (2-5/8” ID) production test string in the exploration well. The main test string data gathering components were:

• Downhole shut-in valve (IRDV) to minimise the impact of wellbore effects on recorded reservoir pressure data

• Surface Pressure Readout facility to minimise rig time by reducing the buildup period to the minimum required to obtain significant reservoir pressure data

• A total of 8 Quartz memory gauges (installed in pairs for data backup) located in the test string as follows:

a) Upper Gauges: Two pairs inside gauges above the IRDV test tool to record pressure in the tubing

b) Middle Gauges: One pair inside gauges installed below IRDV test tool and above top perforations

c) Lower Gauge: Two inside gauge installed below bottom perforations

• SCAR-B sampling module to allow up to 8 single phase downhole samples to be collected at downhole pressure and temperature